Exhaust gas recirculation system for internal combustion engines

ABSTRACT

Apparatus for electronically controlling exhaust gas recirculation in an internal combustion diesel engine to reduce nitrogen oxide emissions. An exhaust gas recirculation control valve is driven by a feedback circuit which compares the valve position with the engine load. A maximum amount of exhaust gas is recirculated under a no-load condition. As the load increases to 100%, the amount of recirculated exhaust gas is decreased down to zero percent. The amount of recirculated exhaust gas also may be affected by the engine speed to minimize smoke in the engine exhaust.

I United States Patent 11 1 Young et a].

[451 Oct. 28, 1975 EXHAUST GAS RECIRCULATION SYSTEM FOR INTERNALCOMBUSTION ENGINES [75] Inventors: Richard N. Young, Richmond;

Larry 0. Gray, Greens Fork, both [21] Appl. No.: 447,573

3,807,376 4/1974 Glockler et a1. 123/119 A Primary Examiner-Wendel1 E.Bums Assistant Examiner-David D. Reynolds Attorney, Agent, or FirmOliver E. Todd, Jr.

[57] ABSTRACT Apparatus for electronically controlling exhaust gasrecirculation in an internal combustion diesel engine to reduce nitrogenoxide emissions. An exhaust gas recirculation control valve is driven bya feedback cir- 52 US. Cl. 123/119 A; 123/119 A Chit which Compares thevalvfi Position with the 51 Im. cl. F02M 25/06 s lofld- A maximum amountof exhaust gas is recir- [58] Field of Search 123/119 A culated under a(How condition AS the load creases to 100%, the amount of recirculatedexhaust [56] Ref Cited gas is decreased down to zero percent. The amountof UNITED STATES PATENTS recirculated exhaust gas also may be affectedby the engine speed to minimize smoke in the engine ex- 2,456,2l312/1948 Pele 123/119 A haust 3,636,934 l/l972 Nakajima et al....;123/119 A 3,799,133 3/1974 Frank 123/119 A 10 Claims, 5 Drawing Figures/.5 f L v ENGINE /6 /Z /4 f I EXHAUST SENSORS I g I v VALVE Ay DR iv 2l0 MOTOR uscrnomc 1 Z 0 CONTROLLE R VALVE POSITION SENS US. PatentOct.28, 1975 sheet 1 on 3,915,134

.3 W: NGINE W f/a /Z EXHAUST SENSORS A;

VALVE DRIVE M ELECTRONIC OTOR /7 CONTROLLER VALVE Pas/now SENSOR 2/ I I1 I g 60 .';50& 5 ABOVE H RRM. 30% Q EZf Lg, BELOW /450R.RM. m I i I 425 50% yo /o PER CENT FULL LOAD.

US. Patent 06. 28, 1975 Sheet 2 f 2 3,915 134 26 f2; OSCILLATOR E (BKHZ)/27 I /0 v n/0011101012 e #52 5 .5 .5 7 /Z/ 37 m K42 43 E wDfMaomw/zE285 L R 36 )W I M 4/ 4 /4 T BIPOLAR POWER SWITCH OSCILLATOR E Z\] (3 KH z) 4 Low P088 FILTER 5 7 62 60 6/ 63 64 F Q E DEMODULRTOR $.1 83 64 Z07% L66 L66 I BIPOLAR 7a POWE R I swrrcu FREQUENCY I: TO VOLTAGECONVERTER w TIE 5- EXHAUST GAS RECIRCULATION SYSTEM FOR INTERNALCOMBUSTION ENGINES BACKGROUND OF THE INVENTION This invention relates tocontrolling exhaust gas emissions from internal combustion dieselengines and in particular to controlling the emission of oxides ofnitrogen in such exhaust gas. The worldwide population increase and theuncontrolled increased use of mechanization in everyday living hascaused increased concern about the environment. Governments have onlyrecently been regulating to protect the environment from pollutants suchas those produced by internal combustion engines. The exhaust frominternal combustion enginesconsists of various constituents includingfully oxidized products of combustion such as carbon dioxide and'waterplus undesirable pollutants such as partially oxidized, cracked andother hydrocarbons, carbon monoxide, oxides of nitrogen and traces ofmiscellaneous other pollutants. The carbon dioxide and water emissionsare unharmful. However, the other exhaust emission constituents areconsidered highly undesirable.

It is generally understood that the presence of nitrogen oxides inengine exhaust is determined by the combustion temperature. An increasein combustion temperature causes an increase in the amount of nitrogenoxides present in the engine exhaust. It is therefore desirable tocontrol the combustion temperature to limit the oxides of nitrogenpresent in the exhaust of an internal combustion engine. One methodsuggested in the prior art for limiting or controlling the combustiontemperature has been to recirculate a portion of the exhaust gas back tothe engine air intake. Since the exhaust gas is low in oxygen, this willresult in a richer combustion mixture which will burn at a lowertemperature. The lower combustion temperature will, in turn, reduce theamounts of nitrogen oxides produced during combustion.

The operating conditions which result in the highest combustiontemperatures depend upon the type of internal combustion engine. In aspark-ignited gasoline engine, for example, the combustion temperaturewill be at a low point during idle. The temperature will also be at alow point atwide open throttle since engines of this type will normallyhave a rich fuel mixture under this condition. Ideally, there should beno exhaust gas recirculation while the engine is idling. From an idle,the amount of recirculated exhaust gas should increase up to a partialload condition and decrease from such partial load condition to a wideopen throttle. In a diesel engine, on the other hand, a maximumcombustion temperature occurs during a no-load condition at idle.Therefore, it is desirable to have a maximum amount of exhaust gasrecirculation during idle and to decrease this amount as the loadincreases to 100% of the rated load.

Various types of controls for exhaust gas recirculation have beensuggested in the prior art. US. Pat. No. 2,456,213 which issued on Dec.14, 1948 to Plec, for example, teaches an early type control system foruse with a diesel engine. In this system, the recirculated exhaust gasflows through two separate series connected valves. One valve ispneumatically controlled in response to the intake manifold vacuum andthe other valve is mechanically controlled in response to engine speed.The net result of the twocontr'ols is that the recirculated exhaust gasis controlled in response to engine load, which is a function. of bothintake vacuum and engine speed. In another prior art recirculationcontrol system, such as is shown in US. Pat. No. 3,703,164 which issuedon Feb. 19, 1971 to Weaving, a third valve is provided for each cylinderfor introducing exhaust gas directly into the cylinder. A mechanicalvalve is provided in series between the engine exhaust and all of thevalves which introduce the exhaust gas into the cylinders to control theamount of exhaust gas recirculated. Various systems have also beenadapted to spark-ignited internal combustion engines. However, each ofthe prior art recirculation systems has incorporated a mechanicalcontrol operating from devices such as cams, centrifugal speed sensors,pneumatic vacuum or pressure sensors, and the like. Although prior artsystems reduce the nitrogen oxide components in engine exhaust gas, theygenerally will not produce an accurate control resulting in a minimumamount of nitrogen oxide under all operating conditions and all usemechanical controls which can be unreliable.

SUMMARY OF THE INVENTION According to the present invention, anelectronic control is provided for exhaust gas recirculation in internalcombustion engines. The control is particularly suitable for meeting thevarious requirements of a diesel engine to reduce nitrogen oxideemissions. The control includes a sensor which measures the load on theengine. The engine load signal is used as a primary control over theposition of an exhaust gas recirculation valve. Preferably, a feedbackcircuit is provided for sensing the position of the exhaust gasrecirculation valve. The valve position is compared with the engine loadfor generating a signal which operates a valve drive motor. The controlcircuit is also provided with means for maintaining the recirculationvalve wide open or in a maximum recirculation condition whenever theload is at a minimum or the engine is idling.

The control may also be modified to further increase its efficiency inreducing nitrogen oxide and other emissions. An engine speed sensor maybe provided for modifying at higher engine speeds the rate at which theexhaust gas recirculation valve is closed under increasing loadconditions. The speed sensor may further be adapted to assure that therecirculation valve is closed when a predetennined maximum engine speedis exceeded and the load on the engine is simultaneously at a minimum,as when the engine is coasting at a high speed. Such controls will havethe advantage of not only minimizing the nitrogen oxide emissions, butalso minimizing the amount of smoke produced by the engine which,although not considered highly harmful, is also undesirable.

Accordingly, it is a preferred object of the invention to provide anelectronic control over exhaust gas recirculation in internal combustionengines to minimize nitrogen oxide emissions.

A further object of the invention is to provide an improved exhaust gasrecirculation system for diesel engines.

Still another object of the invention is to provide an improved exhaustgas recirculation control for diesel engines which operates at maximumefficiency under various conditions of load and engine speed forreducing nitrogen oxide emissions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a'schematic block diagram ofan internal combustion engine incorporating an exhaust gas recirculationsystem constructed in accordance with the present invention;

FIG. 2 is a detailed schematic block diagram of an exhaust gasrecirculation control constructed in accordance with a first embodimentof the invention;

FIG. 3 isa graph showing typical signals generated for operating thedrive motor for the exhaust gas recirculation valve,

FIG. 4 is a chart showing typical operating characteristics for a secondembodiment of a control for an exhaust gas recirculation systemaccording to the present invention; and

FIG. 5 is a detailed schematic block diagram of the second embodiment of'an exhaust gas recirculation control according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning now to the drawings andparticularly to FIG. l,a block diagram is shown for an exhaust gasrecirculation system embodying'the'principles of the present invention.The system 10 is shown connected to'a conventional diesel engine 11which, for example, may be of the type used in-trucks, constructionmachinery and the like. The diesel engine 11 is provided with an :airintake 12 and an exhaust gas outlet 13. The exhaust gas outlet 13 isconnected through a valve 14 to an exhaust pipe 15 which leads to theatmosphere. The ex haust pipe 15 may include a sound muffling system(not shown). The valve 14 includes an adjustable flap 16 which diverts acontrolled portion of the exhaust gas through a return pipe 17 to theair intake 12.

One or more sensors 18 are connected to the engine 11 for measuring atleast the engine load and, in a preferred embodiment, also the enginespeed. The output of thesensors 18 is applied to an electroniccontroller 19 which drives a motor 20 for positioning the valve flap l6.A feedback circuit is preferably provided for assuring a positiveoperation of the exhaust gas recirculation system 10. The feedbackcircuit includes a valve position sensor '21 which applies a signal tothe electronic controller 19 which corresponds to the position of thevalve flap l6. The electronic controller 19 is designed to compare thesignal corresponding to the actual position of the valve flap 16 with adesired position as determined by the engine load conditions monitoredby the sensors 18 and to use a resultant signal for driving the valvedrive motor 20. Generally, the electronic controller 19 drives the motor20 to position the valve flap 16 for permitting a predetermined maximumamount of exhaust gas recirculation under a no-load condition and tolinearly decrease the amount of exhaust gas recirculation by closing thevalve flap 16 as the engine load increases until at 100% rated loadsubstantially no exhaust gas is recirculated through the return pipe 17.The electronic controller 19 is also provided with means for assuringthat the valve flapl6 is at its maximum open position for recirculatinga maximum amount of exhaust gas whenever either the engine load or theengine speed is less than a predetermined amount. This will assure that.the valve flap 16 will remain open during idle of or minimum load onthe engme.

Referring now to FIG. 2, a schematic block diagram is shown for theexhaust gas recirculation system 10 with details of connections: for anelectronic controller 19 for perfomiing the above-described functions.As previously indicated, it is necessary to sense the load on theengine. For a diesel engine, the load may be readily measured from theposition of the rack which controls the amount of fuel injected intoeach cylinder. The rack position may be measured by various conventionalmanners. For example, a linear voltage differential transformer may beconnected to have a core move by the rack. However, this arrangement hasthe disadvantage of placing a load on the rack which may affect theengine operation, particularlywhere the rack position is adjusted by asensitive 'govemor. Preferably, the rack position is determined by aproximity detector or other device which need not be connected directlyto the rack.

As shown in FIG. 2, the load sensor 18 consists of a proximity probe 25which senses the location of a cam surface 26 carried by the rack (notshown). 'An oscillator 27 is connected to provide a relatively lowfrequency alternating current signal to an input of the proximity probe25. The alternating current signal may, for example, be on the order of3 kI-Iz., although the frequency is not critical and will depend uponthe probe design. The proximity probe 25 will then have an output 28 ofthe same frequency as that of the oscillator 27 and of a magnitude whichis inversely dependent upon the spacing from the cam 26.

J The output 28 from the probe 25 is connected through a bufferamplifier 29 to a demodulator 30 which may, for example, consist of ahalf wave rectifier.

The demodulator 30 has an output 3l which is applied through a low passfilter 32 to one input of a compare circuit 33. The low pass filter 32should have a sufficiently low cut-off as to filter any enginevibrations present on the cam 26. The low pass filter 32 is designed toonly partially filter'the demodulator output 31, leaving a ripple on thesignal applied to the compare circuit 33 which is of the same frequencyas the output from the oscillator 27. The output 31 from the demodulator30 is also connected through a diode 34 and a variable resistor 35 toground. The variable resistor 35 may be used for adjusting the gain orload span over which the exhaust gas recirculation valve 14 is operated.I

The output from the oscillator 27 is also connected to the valveposition sensor 21 which consists of a proximity probe 36. A cam 37 isconnected to be driven by the valve drive motor 20 along with theadjustable valve flap 16. The proximity probe 36 senses the spacing tothe cam 37 and generates an output 38 which is inversely proportional tosuch spacing. Thus, the proximity probe output 38 will be of the samefrequency as the output from the oscillator 27 and will increase invalue as the cam 37 approaches the proximity probe the demodulator 40for establishing a zero point for the system 10. The output of thedemodulator 40 is connected through a low pass filter 42 to a secondinput of the compare circuit 33 for comparison with the output from thelow pass filter 32. In a typical diesel engine,- the peak enginevibration is on the order of between Hz. and 50 Hz. Such vibrations maycause the valve flap 16 to flutter and therefore should be filtered fromthe output 38 from the proximity probe 36. It has been found that acut-off frequency of 33 Hz. for the low pass filter 42 is effectiveforapplying a substantially constant signal to the compare circuit 33.This signal has a magnitude which is'determined by the position of thevalve flap 16.

The output of the compare circuit 33 is applied through an amplifier 43to a bipolar power switch 44 which controls power to the valve drivemotor 20. The operation ofthe compare circuit 33 and the bipolar powerswitch 44 for operating the motor 20 may 'be more clearly understood byreferring now to FIG. 3. Graph A in FIG. 3 shows typical inputsappearing on the comparator 33. The low pass filter 32 applies a signal48 to the comparator 33 while the low pass filter 42 applies a signal ofthe type shown as 49a, 49b or 49c to the comparator 33. The comparator33 may consist of a Schmitt trigger which acts as a threshold detectorand has one of two outputs, depending upon which of its two inputs ishighest. If during the majority of the time the valve position signal isabove or nearest the highest ripple level of the load signal 48 as shownat 49a, then a signal of the type shown in FIG. 3B is applied to thebipolar power switch 44; If the ripple level of the load signal 48 is50% above and 50% below the valve position signal as shown at 49b, thena signal of the type shown in FIG. 3C is applied to the bipolar powerswitch 44. If the majority of the ripple in the engine load signal 48 isabove the valve position signal, as shown at 490, then a signal of thetype shown in FIG. 3D is applied to the bipolar power switch 44. Thebipolar power switch 44 will apply a signal to the valve drive motor 20similar to those shown in FIGS. 3B, C and D. These signals alternatebetween equal positive and negative voltages. If the output of thebipolar power switch 44 is of the type shown in FIG. 3C, then there willbe a 50% duty cycle and the motor 20 will stand still since the motor 20is not capable of following the three kilohertz pulses and thereforewill not oscillate. However, if a comparison of the valve position andthe load indicates that the valve should be opened or closed, the dutycycle will change from that shown in FIG. 3C towards one of those shownin FIGS. 3B or 3D. When the power applied to the motor 20 is of theformat shown in FIG. 33, a negative input will be applied to the motor20 for a greater period of time than a positive input and the motor 20will be driven in one direction. When the power applied to the motor 20is changed to the format shown in FIG. 3D, the signal on the motor 20will be positive for a greater time than it is negative and the motorwill be driven in the opposite direction. As the valve flap 16 is drivento a position satisfying the demands of the engine load, the duty cycleof the power applied to the motor 20 will approach 5050 and the motortorque and speed will decrease.

In general, the valve 14 should be operated in a linear fashion withlinear load changes. It is desirable to have maximum recirculation ofexhaust gas at no-load and to have a minimum recirculation at 100% ratedload with the amount of recirculation varying linearly be? tween thesepoints. Although the valve 14 may be of a linear type, it is lessexpensive to use a non-linear valve. Non-linearitiesin the, operation ofthe valve 14 may then be compensated for by providing an appropriatelyshapedsurface on the cam 37 which is rotated with the valve flap l6. 1 1

As previously discussed, it is desirable to maintain maximum exhaust gasrecirculation when the engine is supplying a minimum load. This may beaccomplished by the addition of a second comparator 50 to the circuit ofFIG. 2. The comparator 50 compares the output of the low;pass filter 32with a fixed voltage obtained from a potentiometer 51. The potentiometer51 is used for establishing a trip point at which the recirculationvalve 14 is driven toa fully open condition. When the engine fuel rackmoves below the trip point, the output from the comparator 50 changes.This output is applied to the comparator 33 along with the valveposition sig nal fromthe low pass filter 42. The comparator 33 will thenhave a constant output regardless of its input from the low pass filter32 and will cause the motor 20 to drive the valve 14 to itsfully openstate.

By adding additional controls over the positioning of the exhaust gasrecirculation valve 14, the exhaust gas recirculation system 10 may bemade even more efficient. For example, adiesel engine is typicallyoperated in the range of 1,800 to 2,800 rpm. If the engine is used, forexample, in a truck, the maximum normal op-. erating speed may beexceeded when the truck is coasting down a hill. During coasting, thedriver will normally let up on the accelerator. Since the rack indicatesa no-load condition, the exhaust gas recirculation valve 14 willnormally be driven fully .open. Underthese conditions, the engine mayemit smoke when the driver again hits the accelerator. Therefore, acontrol may be added to drive the exhaust gas recirculation valve 14 toa'fully closed position when the engine is under a noload condition andthe engine speed exceeds a preselected maximum speed, such as 3,000 rpm.A control may also be provided to drive the exhaust gas recirculationvalve 14 to a fully open position at a preselected minimum engine speed,such as 1,200 rpm, regardless of the load on the engine. Still anothercontrol may be provided to minimize smoke in the normal operating range,which, although being appreciably less harmful than nitrogen oxideemissions, is undesirable. It has been determined that less exhaust gasrecirculation can be tolerated at lower engine speeds than at higherengine speeds to prevent the engine from smoking. This is due primarilyto the fact that at higher engine speeds a considerably higher volume:of air passes through the engine. Therefore, a control may also be addedto shift the linear curve on which the exhaust gas recirculation valve14 is operated in response to a predetermined engine speed. For example,smoking may be minimized if the exhaust gas recirculation valve 14recirculates be tween 35% and 0% of the exhaust gas when the engine isoperating between zero load and full load below 1,950 rpm and torecirculate between 50% and 0% of the exhaust gas when the engine isoperatingbetween no-load and full load above 1,950 rpm. This type ofoperation of the exhaust gas recirculation valve 14 is shown in thegraph in FIG. 4'.

Turning now to FIG. 5, a detailed block diagram is shown for apparatus52 including the above-described controls. The apparatus 52 includes anoscillator 53 having an output applied through a' buffer amplifier 54 tothe inputs of proximity probes 55 and 56. The proximity probe 55 ismounted to measure the position of a cam or lobe 57 carried by the fuelrack (not shown) which controls the fuel injectors in a diesel engine.The output of the proximity probe 55 is in the form of an alternatingcurrent signal having a level inversely proportional to the spacing tothe cam 57 and the same frequency as the oscillator 53. This output isapplied through an amplifier 58 to a demodulator 59. The output of thedemodulator 59 is filtered by means of a low pass filter 60 and appliedto one input of a comparator 61. The gain of the signal applied to thecomparator 61 may be controlled by means of a diode 62 and a variableresistor 63 connected between the input of the low pass filter 60 andground in a manner similar to that described above for FIG. 2.

The proximity probe 56 is connected to measure the spacing to a cam 64which is driven by the valve drive motor 20 along with the flap 16 ofthe exhaust gas recirculation valve 14. The alternating current outputfrom the proximity probe 56, which is indicative of the position of thevalve 14, is applied through an amplifier 65 to a demodulator 66. A DCvoltage is applied from a potentiometer 67 to the demodulator 66 forzeroing the apparatus 52. The output from the demodulator 66 is appliedthrough a low pass filter 68 to a second input of the comparator 61. Theoutput of the comparator 61, which alternates between two levels, isapplied through an amplifier 69 and a bipolar switch 70 for driving thevalve motor 20. The operation of the circuitry described so far for FIG.is the same as that described above for the circuitry shown in FIG. 2.However, a number of additional controls are added for further reducingboth smoke and nitrogen oxide emissions.

The additional controls added in FIG. 5 require a signal indicative ofthe speed at which the engine is running. Therefore, a lobed cam 75 ismounted to be driven by the engine. A magnetic pickup 76 is positionedadjacent the lobed cam 75 for generating a pulse train having afrequency proportional to the engine speed. The output of the magneticpickup 76 is applied to a frequency-to-voltage converter 77 whichgenerates a DC output 78 having a voltage level proportional to theengine speed.

The converter output 78 is applied to one input of a comparator 79. Asecond input to the comparator 79 is a fixed voltage determined by apotentiometer 80. The comparator 79 will have one of two outputs whichdepend upon whether or not the converter output 78 is above or below thevoltage set by the potentiometer 80. The potentiometer is adjusted suchthat the output from the comparator 79 will change levels at arelatively low engine speed, such as 1,200 rpm. The output from thecomparator 79 is applied along with the output from the low pass filter60 to the comparator 61. When the output from the comparator 79 changesdue to the engine dropping below the preset speed, a signal is appliedto the comparator 61 to drive the valve 14 to a fully opened condition,allowing a maximum amount of exhaust gas recirculation regardless of theposition of the engine fuel rack. When the preset speed is exceeded, thecomparator 79 will not affect operation of the exhaust gas recirculationvalve 14. Thus, the magnetic speed pickup 76, the converter 77 and thecomparator 79 function to assure that the valve 14 is fully openedwhenever the engine is idling, regardless of load on the engine.

As previously indicated, another desirable operating condition is tohave the valve 14 maintained in a fully closed position whenever thespeed of the engine exceeds it normal operating range and simultaneouslythe fuel injection rack is at a minimum or low load setting. This isaccomplished by means of a pair of comparators 81 and 82 and an AND gate83. The output 78 from the converter 77, which is proportional to enginespeed, is applied to one input of the comparator 81. A potentiometer 84applies a fixed voltage to the second input of the comparator 81 fordetermining the engine speed at which the output from the comparator 81changes. Typically, the potentiometer 84 will be adjusted such that theoutput from the comparator 81 changes levels when the engine speedslightly exceeds its normal operating range, such as 3,000 rpm. Theoutput from the comparator 81 is applied to one input of the AND gate83. The comparator 82 has one input which is connected to the output ofthe low pass filter 60, which output is indicative of the position ofthe fuel injection rack, and a second input connected to a fixed DCvoltage source such as a potentiometer 85 connected between a voltagesource and ground. The potentiometer 85 is adjusted such that the outputof the comparator 82 will change when the fuel injection rack ispositioned for a minimum load. When the output of the comparator 81indicates that the engine speed has exceeded the preselected maximumvalue and the output of the comparator 82 indicates that the fuelinjection rack is at a predetermined minimum load position, the AND gate83 will apply a signal to the comparator 61 for causing the motor 20 todrive the valve to a fully closed position. Thus, there will be minimalexhaust gas recirculation under these conditions. This will prevent theengine from emitting a cloud of smoke when the engine is againaccelerated.

The output 78 from the converter 77 is also connected to a thirdcomparator 86. A potentiometer 87- applies a fixed DC voltage to asecond input of the comparator 86. The potentiometer 87 is adjusted suchthat the output from the comparator 86 changes when the engine passesthrough a preselected midrange speed, such as 1,950 rpm. When thispreselected speed is exceeded, the comparator 86 sinks current from anattenuator 88 and reduces the oscillator signal to the proximity probe56, thereby shifting the linear curve along which the valve 14 isoperated. As previously discussed, two typical curves are shown in thegraph of FIG. 4 for the operation of the valve 14. When the engine isrun below 1,950 rpm, the maximum exhaust gas recirculation for oneengine was found to be about 35% for no-load for providing a goodbalance between smoke and nitrogen oxide pollutants. Thus, at idle orany speed below 1,950 rpm, about 35% of the exhaust gas is recirculatedfor a no-load condition of the engine. As the load on the engineincreases to of its rated value, the recirculated exhaust gas islinearly decreased to 0% or to some other predetermined low level. Athigher engine speeds, additional exhaust gas may be recirculated withouthaving adverse affects on engine smoking. Therefore, above 1,950 rpm,about 50% of the exhaust gas is recirculated for a no-load condition andthis is decreased linearly to 0% recirculation for a 100% load. It will,of course, be appreciated that these percentages may vary for any givenengine as well as the engine speed at which the curves are shifted.Furthermore, there may be several shifts in the recirculation curve fordifferent engines or the curve may vary continuously with engine speed.

Although not shown in the drawings, it will be appreciated that variousother modifications may be made in the above-described exhaust gasrecirculation system. For example, it may be desirable to protect themotor when the valve is driven to a fully open or a fully closedposition. This could be accomplished, for example, by the addition ofcircuitry which places a limit on the maximum time at which the motorcan be operated at full power in either direction and to then reduce thecurrent. This will not only protect the motor when it is driven fullyclosed or fully opened, but is would also protect the motor should thevalve become jammed.

, It should also be appreciated that the abovedescribed exhaust gasrecirculation system may also be adapted for use with other types ofinternal combustion engines, such as spark-ignited gasoline engines. Theconditions under which the exhaust gas recirculation is controlled will,of course, be modified for the requirements of any particular engine.For example, a sparkignited gasoline engine may require maximumrecirculation at a mid-speed range rather than at minimum load and idleconditions.

Various other modifications and changes may be made to the presentinvention without departing from the spirit and the scope of thefollowing claims.

What we claim is:

1. For an internal combustion engine having an air intake and anexhaust, an exhaust gas emission control system comprising, incombination, valve means for selectively recirculating a portion of theexhaust gas to the air intake, said valve means moving between a firstposition wherein a predetermined maximum amount of the exhaust gas isrecirculated and a second position wherein a predetermined minimumamount of the exhaust gas is recirculated, means for generating anelectric signal indicative of a load on the engine, and electrical meanspositioning said valve means at predetermined intermediate positionsbetween said first and second positions in response to such signal forreducing the oxides of nitrogen present in the engine exhaust gas.

2. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 1, wherein said valve positioning meansincludes electrical control means for moving said valve means to saidfirst position when the load on the engine is less than a predeterminedvalue and for moving said valve means progressively towards said secondposition as the load progressively increases above such predeterminedvalue.

3. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 2, and further including means for movingsaid valve means to said first position when the engine is running atless than a predetermined minimum speed.

4. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 3, and including means responsive to suchelectric signal and to the engine speed for moving said valve means tosaid second position when the load on the engine is less than apredetermined value and simultaneously the engine exceeds apredetermined speed.

5. An exhause gas emission control system for an internal combustionengine, as set forth in claim 4, and further including means responsiveto the speed of the engine for modifying said predetermined maximumamount of exhaust gas recirculated when said valve means is in saidfirst position.

6. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 1, including means for generating anelectric signal indicative of the position of said valve means, andwherein said valve positioning means includes an electric motorconnected to drive said valve means, means for comparing said engineload signal and said valve position signal and means responsive to suchcomparison for controlling said valve drive motor.

7. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 6, wherein said valve positioning meansfurther includes means responsive to said engine load signal forgenerating a third signal when the load on the engine is less than apredetermined minimum, and means for causing said valve drive motor tomove said valve means to said first position in response to said thirdsignal.

8. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 6, and further including means forgenerating an electric sig nal when the engine is running at less than apredetermined minimum speed, and means for causing said valve drivemotor to move said valve means to said first position in response tosaid minimum speed signal.

9. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 6, and further including means forgenerating an electric signal when the load on the engine is less than apredetermined minimum, means for generating an electric signal when theengine speed exceeds a predetermined maximum, and means responsive tothe simultaneous occurrence of said minimum load signal and said maximumspeed signal for causing said valve drive motor to move said valve meansto said] second position.

10. An exhaust gas emission control system for an internal combustionengine, as set forth in claim 6, and further including means forgenerating a third electrical signal when the engine exceeds apredetermined speed, and means responsive to said third signal forincreasing said predetermined maximum amount of exhaust gas recirculatedwhen said valve means is in said first positron.

1. For an internal combustion engine having an air intake and anexhaust, an exhaust gas emission control system comprising, incombination, valve means for selectively recirculating a portion of theexhaust gas to the air intake, said valve means moving between a firstposition wherein a predetermined maximum amount of the exhaust gas isrecirculated and a second position wherein a predetermined minimumamount of the exhaust gas is recirculated, means for generating anelectric signal indicative of a load on the engine, and electrical meanspositioning said valve means at predetermined intermediate positionsbetween said first and second positions in response to such signal forreducing the oxides of nitrogen present in the engine exhaust gas.
 2. Anexhaust gas emission control system for an internal combustion engine,as set forth in claim 1, wherein said valve positioning means includeselectrical control means for moving said valve means to said firstposition when the load on the engine is less than a predetermined valueand for moving said valve means progressively towards said secondposition as the load progressively increases above such predeterminedvalue.
 3. An exhaust gas emission control system for an internalcombustion engine, as set forth in claim 2, and further including meansfor moving said valve means to said first position when the engine isrunning at less than a predetermined minimum speed.
 4. An exhaust gasemission control syStem for an internal combustion engine, as set forthin claim 3, and including means responsive to such electric signal andto the engine speed for moving said valve means to said second positionwhen the load on the engine is less than a predetermined value andsimultaneously the engine exceeds a predetermined speed.
 5. An exhaustgas emission control system for an internal combustion engine, as setforth in claim 4, and further including means responsive to the speed ofthe engine for modifying said predetermined maximum amount of exhaustgas recirculated when said valve means is in said first position.
 6. Anexhaust gas emission control system for an internal combustion engine,as set forth in claim 1, including means for generating an electricsignal indicative of the position of said valve means, and wherein saidvalve positioning means includes an electric motor connected to drivesaid valve means, means for comparing said engine load signal and saidvalve position signal and means responsive to such comparison forcontrolling said valve drive motor.
 7. An exhaust gas emission controlsystem for an internal combustion engine, as set forth in claim 6,wherein said valve positioning means further includes means responsiveto said engine load signal for generating a third signal when the loadon the engine is less than a predetermined minimum, and means forcausing said valve drive motor to move said valve means to said firstposition in response to said third signal.
 8. An exhaust gas emissioncontrol system for an internal combustion engine, as set forth in claim6, and further including means for generating an electric signal whenthe engine is running at less than a predetermined minimum speed, andmeans for causing said valve drive motor to move said valve means tosaid first position in response to said minimum speed signal.
 9. Anexhaust gas emission control system for an internal combustion engine,as set forth in claim 6, and further including means for generating anelectric signal when the load on the engine is less than a predeterminedminimum, means for generating an electric signal when the engine speedexceeds a predetermined maximum, and means responsive to thesimultaneous occurrence of said minimum load signal and said maximumspeed signal for causing said valve drive motor to move said valve meansto said second position.
 10. An exhaust gas emission control system foran internal combustion engine, as set forth in claim 6, and furtherincluding means for generating a third electric signal when the engineexceeds a predetermined speed, and means responsive to said third signalfor increasing said predetermined maximum amount of exhaust gasrecirculated when said valve means is in said first position.